For recent years, the number of small and thin electronic devices (hereafter called as small electronic devices) such as a mobile phone and a digital still camera which are equipped with a small imaging optical system has been growing. Most of small electronic devices are equipped with a fixed focus lens, because the space and the depth for the lens are limited. The fixed focus lens can be embedded in a small electronic device without a problem, because the overall length of the fixed focus lens can be controlled to about 5 mm. On the other hand, the overall length of a zoom lens with the zooming ratio of about 3 need to be about 20 mm. Therefore it is almost difficult to embed a zoom lens into a small electronic device in the straight configuration. Instead, the optical axis is usually bent 90 degrees by means of a prism or a mirror in order to embed a zoom lens in a small electronic device the space and the depth of which are limited.
For example, an imaging optical system is proposed in Reference 1, where the optical axis is bent 90 degrees by means of a prism. In the proposal of Reference 1, a prism with concave surfaces at the both side is used. By forming a concave surface at the front side of a prism, the ray height entering into the front surface of the prism can be kept low and the thickness of the prism can be kept small.
An imaging optical system is proposed in Reference 2, where a concave lens is included before a prism. In the proposal of Reference 2, by including a concave lens before a prism, the ray height entering into the front surface of the prism can be kept low and the thickness of the prism can be kept small, similarly to Reference 1.
In Reference 3, the rotation of a prism by 45 degrees during the storage is proposed to reduce the thickness during the storage. In Reference 4, an optical system without a concave surface at the front surface of a prism and without a concave lens before a prism is proposed.